This invention relates to a weight control system for rapidly and accurately dispensing a predetermined quantity of material into a container.
In high-speed packaging operations where material is batched into individual batches or charges which are then placed in a package on a net weight basis, certain long-standing and difficult to solve problems have been encountered. One of these problems is controlling or minimizing the package weight variance, that is, the excursion of net package weight from a desired predetermined net package weight. There are several reasons for variances in package weight. One reason is the inability of present package filling devices to feed material into a charge container at a constant rate. Considerable attention has been given by the industry to solving this problem, but attempted solutions have not been as successful as desired.
One of the best known methods of dispensing material to form a charge which is then packaged is commercially termed the "bulk-dribble" method. By this method, material is initially dispensed at a relatively high feed rate to form a charge until the weight of the charge reaches a preselected high percentage, for example, 90 percent of its ultimate net weight. This phase of the dispensing operation is termed the bulk feed interval. Once the weight of the charge reaches the preselected percentage of its ultimate weight, the material is dispensed at a lower feed rate. This phase is designated the dribble feed interval and is completed when the weight of the charge reaches the predetermined net weight. By using the "bulk-dribble" technique, the quantity of material in transit between the dispenser and the charge weighing scales is substantially reduced during the dribble feed interval which permits greater control of the ultimate quantity of material dispensed. The quantity of material in transit, however, is subject to random fluctuations which are not readily controllable by the dispensing mechanism. This is particularly so when the material consists of materials of non-uniform size and weight, such as potato chips and other such food items.
Another method of dispensing material to form a charge is disclosed in copending patent application Ser. No. 284,708, entitled "Weight Control System" and assigned to the common assignee herewith. In that package weight control system, the weight of material being dispensed into a hopper is continually monitored and compared with an idealized package weight curve. Thus, as the weight of the charge in the hopper progressively increases, the charge weight is made to follow the profile dictated by the idealized package weight curve. If the weight of the charge exceeds the idealized package weight, the feeder is temporarily shut down. On the other hand, if the weight of the charge is less than the idealized weight, the feeder is energized. By following an optimum profile curve, the weight of the charge is continuously monitored such that an accurate quantity of material is dispensed onto the hopper.
However, each of the aforementioned material dispensing techniques includes an inherent disadvantage. The time lag between the time an incremental amount of product is dispensed into a hopper or package and the time that a scale measures the weight of material therein permits extra amounts of material to be dispensed when, in fact, such extra materials are not required to bring the weight of the charge up to the predetermined desired weight limit. The time lag is composed of two elements, namely, the time required for the dispensed material to come to a state of rest in the hopper and the time required for the scales to advance to a new position of equilibrium so that a signal proportional to the new weight of the charge can be generated. In high-speed packaging operations, the time lag is significant relative to other system time constants and, in fact, limits the overall system response and accuracy. Any innovation in the packaging technology which eliminates or at least decreases this time lag will favorably improve the total system performance of the packaging system.
In view of and to overcome these problems or disadvantages of known prior art systems the present invention has for an object to provide a system for weighing a product into batches with a weight accuracy limit determined only by the product itself and/or the packager's need or desire to package a single or a plurality of product units at a time. Thus, the limit of accuracy is not determined by scale reference time, product settling time, scale hopper accommodations, feed rate, and the like which limit accuracy of all heretofore known systems.